Bioabsorbable polymers and medical devices made from such polymers are known in the art. Conventional bioabsorbable polymers include polylactic acid, poly(p-dioxanone), polyglycolic acid, copolymers of lactide, glycolide, p-dioxanone, trimethylene carbonate, ε-caprolactone, in various combinations, etc. The bioabsorbable polymers are designed to have a chemistry such that the polymers breakdown in vivo and are either metabolized or otherwise broken down, for example by hydrolysis, and excreted from the patient's body. The advantages of utilizing implantable medical devices made from bioabsorbable polymers are numerous and include, for example, eliminating the need for additional surgeries to remove an implant after it serves its function. Ideally when a “temporary presence” of the implant is desired, support can be provided until the tissue heals.
The bioabsorbable polymers used to manufacture medical devices have been on occasion polymeric blends of absorbable polymers and copolymers engineered to provide specific characteristics and properties to the manufactured medical device, including bioabsorption rates, breaking strength retention, and dimensional stability, etc.
There are many conventional processes used to manufacture medical devices from bioabsorbable polymers and polymer blends. The processes include injection molding, solvent casting, extrusion, machining, cutting and various combinations and equivalents. A particularly useful and common manufacturing method is thermal forming using conventional injection molding processes. It is known in this art that manufacturing processes such as thermal injection molding may result in molded parts that have inferior properties, especially, for example, unacceptable dimensional stability, mechanical properties, and retention of mechanical properties with time post-implantation. There are a number of reasons for diminished dimensional stability. They include the presence of residual stresses induced during the manufacturing process. Another reason is if at least one of the polymeric components possesses too low a glass transition temperature, especially if the polymeric component does not easily crystallize after molding.
Therefore, there is a need in this art for novel bioabsorbable polymer blends that can be used in thermal injection molding processes, and other conventional processes, to manufacture bioabsorbable medical devices having superior breaking strength retention, excellent bioabsorption, superior mechanical properties such as stiffness and strength, manufacturability, and superior dimensional stability.
It is known when using thermal injection molding processes that process conditions and design elements that reduce shear stress during cavity filling will typically help to reduce flow-induced residual stress. Likewise, those conditions that promote sufficient packing and uniform mold cooling will also typically tend to reduce thermally-induced residual stress. It is often very difficult, if not nearly impossible, to completely eliminate residual stress in injection molded parts. Approaches that have been employed include: (1) attempting to crystallize the part while still in the mold to increase the mechanical rigidity to resist distortion; and, (2) employing resins having a high glass transition temperature (Tg).
This later case describes the situation wherein chain mobility is only reached at much higher temperatures, thus protecting the part at the moderate temperatures that the part might be expected to endure during ethylene oxide (EO) sterilization, shipping, and storage. Materials possessing high glass transition temperatures may not necessarily possess other characteristics that are desirable such as absorbability. Residual stresses are believed to be the main cause of part shrinkage and warpage. Parts may warp or distort dimensionally upon ejection from the mold during the injection molding cycle, or upon exposure to elevated temperatures, encountered during normal storage or shipping of the product.
There have been attempts in the prior art to address the problem of lack of dimensional stability in medical devices thermally formed from melt blended bioabsorbable polymers. Smith, U.S. Pat. No. 4,646,741, discloses a melt blend of a lactide/glycolide copolymer and poly(p-dioxanone) used to make surgical clips and two-piece staples. The melt blends of Smith provide molded articles possessing dimensional stability; Smith requires that the amount of poly(p-dioxanone) in the blend is greater than 25 weight percent and teaches away from lower amounts. The polymer blends of Smith have disadvantages associated with their use to manufacture medical devices, including: limited stiffness or Young's modulus, shorter retention of mechanical properties upon implantation, greater sensitivity to moisture limiting the allowable open storage time during manufacture, and, although difficult to quantify, more difficult thermal processing.
As mentioned previously, residual stresses are believed to be the main cause of part shrinkage and warpage. It is known that flow-induced residual stresses may have an effect upon a thermally molded polymeric medical device. Unstressed, long-chain polymer molecules tend to conform to a random-coil state of equilibrium at temperatures higher than the melt temperature (i.e., in a molten state). During thermal processing (e.g. injection molding), the molecules orient in the direction of flow, as the polymer is sheared and elongated. Solidification usually occurs before the polymer molecules are fully relaxed to their state of equilibrium and some molecular orientation is then locked within the molded part. This type of frozen-in, stressed state is often referred to as flow-induced residual stress. Anisotropic, non-uniform shrinkage and mechanical properties in the directions parallel and perpendicular to the direction of flow are introduced because of the stretched molecular structure.
Cooling can also result in residual stresses. For example, variation in the cooling rate from the mold wall to its center can cause thermally-induced residual stress. Furthermore, asymmetrical thermally-induced residual stress can occur if the cooling rate of the two surfaces is unbalanced. Such unbalanced cooling will result in an asymmetric tension-compression pattern across the part, causing a bending moment that tends to cause part warpage. Consequently, parts with non-uniform thickness or poorly cooled areas are prone to unbalanced cooling, and thus to residual thermal stresses. For moderately complex parts, the thermally-induced residual stress distribution is further complicated by non-uniform wall thickness, mold cooling, and mold constraints.
It should be noted that a common, conventional method of sterilization is exposure to ethylene oxide gas in a sterilization process cycle. Absorbable polymeric devices are frequently sterilized by exposure to ethylene oxide (EO) gas. EO can act as a plasticizer of lactide-glycolide copolymers, and can lower the Tg slightly; this may result in ‘shrinkage’ and/or ‘warpage’ of an injection-molded part, especially when exposed to temperatures higher than the Tg. This adds additional processing and handling challenges when using lactide-glycolide polymeric materials for absorbable medical devices. It should be noted that the EO sterilization process not only exposes the part to EO gas, it also exposes the part to elevated temperatures. This usually requires treatment at slightly elevated temperatures. Because EO can act as a plasticizer of synthetic absorbable polyesters, the problems of shrinkage and warpage and general dimensional instability are often exacerbated.
There are a number of processing methods conventionally used to reduce or eliminate shear stresses during processing. Process conditions and design elements that reduce shear stress during cavity filling will help to reduce flow-induced residual stress. Polymeric parts are often heat treated (thermally annealed) to alter their performance characteristics. The reason for the heat treatment processing is to mature the morphological development, for example crystallization and/or stress relaxation. If done successfully, the resulting part may exhibit better dimensional stability and may exhibit better mechanical strength.
Injection molded parts ejected from the injection molding machine that are not already distorted, can be cooled/quenched to room temperature and may appear to be dimensionally sound. Stresses, however, are usually still present and can drive distortion any time the polymer chains are allowed to mobilize. As previously described, this can happen with an increase in temperature or exposure to a plasticizer such as EO gas. In order to overcome this potential driving force for dimensional distortion, a number of strategies have been taken; these include (thermal) annealing.
If the part can be dimensionally constrained, thermal annealing can be employed towards two goals: one is to attempt to reduce the amount of molecular orientation in the polymer chains, also known as stress reduction; and, another is to increase the crystallinity in the part to increase the mechanical rigidity to resist distortion.
With some polymers that readily crystallize, one might be able to crystallize the part while it is still in the mold, but this is an unusual situation. Here the mold cavity not only acts to define the shape of the part, it can act to restrain the shape of the part during the crystallization process. With more-difficult-to-crystallize polymers, the cycle time becomes prohibitively long, and the injection molding process becomes impractical. Thus, the part needs to be ejected from the mold before complete polymer morphology development takes place.
Injection molded parts prepared from semi-crystalline polymers can often be annealed by thermal treatment to increase crystallinity level and complete their polymer morphology development. Often the parts must be physically constrained to avoid the distortion one is attempting to avoid. Once crystallized, the part has increased mechanical rigidity to resist distortion if exposed to normally distorting conditions. Providing suitable physical constraint is often difficult, as it is often labor intensive and economically taxing.
Annealing the ejected part without need for physical constraint is preferred; however what often happens is that the part distorts during the annealing process rendering the part unacceptable for many needs.
It is known in the industry to anneal parts to reduce molded-in-stresses by thermal relaxation. The time and temperature required to relieve stress varies but must often be done below the Tg to avoid gross distortion. Even then the results can vary greatly. It is more difficult to reduce stress levels, without causing distortion, in higher molecular weight resins. It would be relatively easy to reduce molded-in-stresses by thermal relaxation in low molecular weight, high flow, polyesters, as compared to higher molecular weight polyesters.
Regarding the molecular weight of the polymer blend, higher molecular weight usually develops higher stress levels and requires longer times/higher temperatures for stress relaxation. Although this is the case, higher molecular weight is often needed to achieve high mechanical properties and biological performance. This situation often presents a problem for the device manufacturer.
In order to impart more crystallinity to increase mechanical rigidity to better resist distortion, or to reduce molecular orientation in order to lower the driving force for distortion, the parts would ideally be processed by thermal treatment (annealing) at a temperature which does not cause distortion. Unfortunately, due to the nature of the synthetic absorbable polyesters commonly employed, this treatment often needs to be above their glass transition temperature where distortion is nearly impossible to avoid.
Consider for example, polylactide homopolymeric or poly(lactide-co-glycolide)copolymeric devices. The stressed polymer chains of these injection-molded parts will tend to relax and return to their natural state (“random three-dimensional coils”) when heated to or above their glass transition temperatures. This will be observed as warpage, shrinkage or general dimensional deformation. It is a general practice in the industry when producing molded polylactide-based parts, not to anneal them because of this potential deformation. These as-molded polylactide parts are of very low crystallinity, if not outright amorphous or non-crystalline, and will then tend to deform if exposed to temperatures at or above their respective glass transition temperatures. It would be advantageous to be able to anneal such parts to induce crystallinity so that they may develop the high rigidity to remain dimensionally stable under conditions normally encountered during EO sterilization, shipping, and storage.
There are medical applications that require the medical device to display sufficient column strength such as in the case of an implantable staple or a tack. Clearly, for a device having such a requirement with a smaller cross-sectional area, the polymer from which it was formed must be inherently stiff if the tack is to function properly for the intended application.
To achieve higher stiffness in a melt blend of a lactide/glycolide copolymer and poly(p-dioxanone), one needs to minimize the amount of poly(p-dioxanone). Contrary to what Smith teaches, it has been found that dimensional stability can be achieved in parts molded from a blend of a lactide-rich copolymer and poly(p-dioxanone), in which the levels of poly(p-dioxanone) are lower than 25 weight percent. The addition of the poly(p-dioxanone), even at these low levels, enhances the ability to achieve dimensional stability in the final part.
Even though such polymer blends are known, there is a continuing need in this art for novel absorbable polymeric materials that provide a medical device with improved characteristics including stiffness, retained strength in vivo (in situ), dimensional stability, absorbability in vivo, and manufacturability, along with a need for novel medical devices made from such polymeric materials, and novel methods of manufacturing medical devices from such polymeric materials.